1. Field of the Invention
The present invention relates to analysis of radio frequency (RF) nonlinear systems with memory effects, and particularly to a method for memory effects quantification and comparison in RF transmitters and amplifiers.
2. Description of the Related Art
Memory effect” is a distortion characteristic that results in having the instantaneous output of a system to depend on its current input as well as a set of finite preceding inputs.
Memory effect can be caused by the thermal operating characteristics of the semiconductor device. In this case, it is referred to as “thermal memory effect”. When a high peak-to-average power ratio (PAPR) signal having a narrow bandwidth (in the range of few MHz or less) is being transmitted via power amplifier, the transitory temperature rises very quickly when the peak occurs. Thus, for a couple of microseconds, the temperature of the transistor rises. For example if the peak-to-average power ratio of the signal is 6 dB, the heat generated by the transistor is approximately four times higher than average.
This phenomenon causes the instantaneous gain of the power amplifier to vary as a function of the junction temperature. In other words, the gain at a given instant will be function of the junction temperature, which is, in turn, a function of the power levels of the preceding input samples. That is why it is called “memory effect”, because the behavior of the power amplifier is related to the characteristic history of the signal being transmitted. The thermal resistances and thermal capacitances of the semiconductor is a key figure in memory effect.
Memory effect can also be caused by the wideband width of the input signals applied to the power amplifiers or transmitters. In this case, it is commonly labeled as “electrical memory effect”. Electrical memory effects are caused by the frequency response of the biasing circuitries and matching networks of the power amplifier at the baseband, around the carrier frequency as well as its harmonics. Electrical memory effects also cause the output of the system to depend on its current as well as preceding status.
Memory effects are very present in modern communication systems, and they significantly impact the performance of the power amplifier and transmitter. They are difficult to compensate, and thus it is very important to be able to measure them and quantify them to guide the design and optimization of the power amplifier and transmitter.
Two memory effects intensity metrics have been previously proposed to quantify memory effects in transmitters and power amplifiers driven by modulated signals. These metrics consist in calculating the power in the adjacent channel after cancellation of the memory-less nonlinearity of the device under test (DUT). One of the techniques requires linearizing the DUT through memory-less predistortion, and then measuring its output signal to calculate the memory effects intensity. The second metric is derived after cancelling the memory-less nonlinearity of the DUT using memory-less postcompensation, and then measuring the obtained output signal to calculate the memory effects intensity. A more direct approach is sought.
Thus, a method for memory effects quantification and comparison in RF transmitters and amplifiers solving the aforementioned problems is desired.